As with many things in life, the answer is "it depends" ;)
For me, the answer is: you don't.
I enjoy learning, so I don't try to limit myself. Rather, I try to systematize it a bit. If I start learning new thing, I add it to my list of things I currently learn, and assign a (learning) priority. I try to keep at most 2-3 new subjects I learn (excluding deepening my knowledge of the topics I already know). If I stumble at something new, while already learning few new things, I just note it down to check up later. (a good example is my current learning of haskell: I decided I want to learn it few years ago, but only about a month ago I actually started learning it).
But if you do not enjoy learning by itself, then maybe you should just think about what you do want, and then focusing on a way to achieve that. Assume everything that's neither a leisure nor a step towards your goal a distraction (even if it's learning a new thing).
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